Heat sensitive recording materials which utilize combinations of electron-donating colorless dyes and electron-accepting compounds are disclosed, e.g., in Japanese Patent Publications 14039/70, 4160/68, and so on.
Characteristics that the heat sensitive materials of the above-described kind should possess include, to say the least, (1) to produce developed image having sufficiently high color density and sufficiently high coloring speed, (2) to generate no fog (e.g., coloring phenomenon upon storage before use), (3) to provide a coloring product having sufficiently high color fastness, and so on. However, materials which perfectly possess these desired properties have not yet been obtained.
In keeping step with the recent speeding-up of heat sensitive recording systems, concentrated study of the above-described characteristics (1) has particularly been made. Specifically, a method of using an oxybenzoic acid ester as an electron accepting compound (Japanese Patent Application (OPI) No. 144193/81 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application.)), a method of using a salicyclic acid ester as an electron accepting compound (Japanese Patent Application (OPI) No. 201693/82), and so on are disclosed. However, the combinations of electron-donating colorless dyes (also referred to hereinafter as a color former) and electron-accepting compounds (also referred to hereinafter as a color developer) which can effect the acquirement of the property (1) are generally inferior with respect to ensuring the properties (2) and (3) and therefore, require some measures to prevent generation of fog and discoloration.
On the other hand, it has already been known that heat sensitive materials which provide coloring products having excellent color fastness and generate appreciably reduced fog can be obtained by combining the color former of triarylmethane type, diphenylmethane type, xanthene type, thiazine type or spiropyran type with the color developer of phenol type, especially bisphenol type. However, those combinations were found to have insufficient coloring speed.